The present invention relates to improvements in the field of elastic waveguides. More particularly, the invention is concerned with an improved single-mode acoustic fiber for propagating acoustic waves in transverse-type modes.
U.S. Pat. No. 3,922,622 issued to G. D. Boyd and L. A. Coldren on Nov. 25, 1975 discloses a form of waveguide for elastic or acoustic waves. Such a waveguide comprises an elongated control liquid or solid core region and an outer solid cladding region enclosing all surfaces of the core region except end surfaces thereof, both regions being composed of materials in which acoustic waves can be propagated. The core and cladding materials are selected so that they focus and contain acoustic wave energy predominantly within the core region. The Boyd-Coldren waveguide is adapted to propagate acoustic waves in radial and torsional modes as well as in longitudinal modes.
Also known in the art is an elastic waveguide for propagating acoustic waves in yet a fourth set of modes, referred to as the shear modes (U.S. Pat. No. 4,077,023 to G. D. Boyd and R. N. Thurston). The shear modes are characterized by a principal particle displacement which is substantially perpendicular to a plane passing through the central longitudinal axis of the core region. The Boyd-Thurston waveguide has solid core and cladding regions where the shear wave velocity of the cladding region is larger than the shear wave velocity of the core region.
In a single shear-type mode acoustic fiber with a core of circular cross-section, such as that disclosed in the Boyd-Thurston patent, polarization of the fundamental shear mode F.sub.11 can be resolved into two orthogonal components F.sub.11X and F.sub.11Y. When the circularity of the core is perfect and is maintained along the fiber length, the propagation coefficient of the modes in the two orthogonal directions are exactly the same. The input state of polarization should be maintained along the fiber such that at the fiber output the polarization of the mode is predominantly in one direction if it so launched at the input.
However, in practice, geometrical distortions such as kinks and bends as well as material inhomogenities in the fiber induce power coupling between the modes in the two orthogonal directions, thus removing the degeneracy between the two modes and making the single mode fiber "bi-modal". As a result, if a linearly polarized shear acoustic wave transducer is used to excite only one of these two modes, some of the power will be coupled to the other mode, thereby altering the polarization state of the shear acoustic waves in the fiber, and the state of polarization at the fiber output will thus be arbitrary. In other words, the polarization of another shear wave transducer arranged at the fiber output to receive the maximum amount of transmitted acoustic energy will be uncertain. This coupling is also environmentally sensitive so that not only is the polarization state at the output different from that at the input, but it may vary in time as well. Moreover, since the shear acoustic wave transducer is generally bonded by epoxy to the fiber, it cannot be easily displaced to coincide with maximum output acoustic energy transmitted at the receiving end of the fiber. Similar alignment problems apply at the transmitting end of the fiber when the latter is used for reflection geometry.